It is known that properties of high speed steels containing alloy elements such as Cr, W and V can be improved by incorporation of nitrogen into the steels (see, for example, Kobe Steel Technical Bulletin, R & D, Vol. 24, No. 3, pages 11 to 15, and Japanese Patent Application Laid-Open Specifications No. 78606/74, No. 49109/75 and No. 49156/75). By the nitriding treatment, a nitride of the type MX or M.sub.6 X (in which M stands for an alloy element and X stands for carbon or nitrogen) is formed, and this nitride is more stable than a carbide of the MC or M.sub.6 C type. Accordingly, the appropriate quenching temperature range is broadened and control of the heat treatment can be facilitated. Further, the temper hardening characteristic is improved and a finer austenite crystal structure can be obtained to improve the mechanical properties. Furthermore, the machinability of the steel can be improved. It is construed that by virtue of these effects, the properties of such high speed steels can be improved by incorporation of nitrogen into the steels.
Most conventional nitrogen containing high speed steels have heretofore been prepared by the smelting process. When the smelting process is adopted for production of nitrogen containing high speed steels, it is necessary to perform complicated steps such as the step of melting steel in a high pressure nitrogen atmosphere or the step of throwing a nitride into molten steel. Further, according to the smelting process, since the amount of nitrogen included in steel is small and it is difficult to form a fine carbonitride and distribute it uniformly in steel, it is impossible to improve the properties to desirable levels.
As a means of overcoming the defects or limitations involved in the smelting process, methods have recently been proposed for obtaining nitrogen containing high speed steels by the powder metallurgical process or the powder forging process. In these methods, by utilizing the fact that powder has a large specific surface area and the fact that a powder sintered body has a porous structure, an optional amount of nitrogen can be included in steel by a simple means, for example, by adding nitrogen in advance to the starting powder or adjusting the heating temperature, the heating time or the nitrogen partial pressure in the treatment atmosphere at the sintering step. It is expected that nitrogen will be finely and uniformly distributed in steels according to these methods.
In conventional nitrogen containing high speed steels obtained by the powder metallurgical process, the machinability is not as highly improved as might be expected. Rather, the machinability is degraded by incorporation of nitrogen into the steel. Accordingly, it is often said that the value of nitrogen containing high speed steels obtained by the powder metallurgical process is questionable. Moreover, several nitrogen containing high speed steels obtained by the powder metallurgical process, which have recently been put into practical use, have exhibited good machinability and good wear resistance in combination. The reason for this has not been elucidated. In particular the relation between amounts of alloy elements which impart excellent machinability to steels and the amount of nitrogen enrichment is not clarified. Therefore, the kinds of steels which are enriched with nitrogen for the production of high speed steels by the powder metallurgical process and which are applicable are drastically limited. For example, Kobe Steel Technical Bulletin, R & D, Vol. 24, No. 3, page 10 discloses that when 0.4 to 0.5% of nitrogen is added to Mo type high speed steels (JIS SKH 9 and modified JIS SKH 55) by the powder metallurgical process, the machinability is remarkably improved. Indeed, improved machinability can be attained by such technique. However, the addition of such a large amount of nitrogen to high speed steels in which the contents of alloy elements are standarized results in disorder of the stoichiometric balance among alloy elements in high speed steels. Therefore, various problems are caused by this technique. For example, the residual austenite content at the quenching step is increased and it is necessary to increase the number of repetitive tempering steps. Accordingly, the size change during heat treatment is enhanced. Moreover, the toughness is degraded and the applicable range of cutting tools of these steels is considerably limited.
The above advantage of the powder metallurgical process in that an optional amount of nitrogen can be included into steel and a fine carbonitride can be distributed uniformly in the steel was observed and consequently research was conducted with a view to improving various properties of high speed steels, especially the machinability, by nitrogen enrichment of high speed steels comprising various alloy components according to the powder metallurgical process. As a result, we found that in order to improve the machinability without degradation of heat treatment characteristics and mechanical properties of high speed steels, a specific relationship must be established among C, N and V as specific ingredients of high speed steels. The present invention has been completed based on this finding.